Such an apparatus has been disclosed by the older German Patent Application No. P 3,831,399.5. In that case there is an outer casting 1, an outlet orifice for an electrolyte at the bottom in the extension 6, an inner casing 3 in the first connection, a first and second connection in the first chamber 4 and in the second chamber 2, a nozzle pipe and a pump 7.
Depending on the position of the piston in the pump 7, electrolyte spurts in the measuring state, that is to say, with the exit orifice closed, first out of the nozzle pipe to the test specimen and then back into the chamber 4 or alternatively, the electrolyte rushes out of the chamber 4 towards the outlet orifice and towards the test specimen and is sucked back again through the nozzle pipe. The frequency of this flow is in the region of one hertz.
Whether it is acidic or basic, electrolyte is corrosive. In normal operation, it also does not escape from the probe. It has been found, however, that contrary to the operating instructions, the probe is lifted during the measuring process. This has the consequence that the electrolyte is very rapidly sprayed out of the outlet orifice, whether it is forced out of the first chamber or whether it is forced out of the second chamber.
The consequences of this can readily be imagined: the electrolyte is lost. The layer is attacked and is consequently perhaps unusable. Above all, however, the electrolyte may enter, for example, the eyes, the nose, the mouth or the like, of the operator and/or get onto his clothing which is then corroded.
Apparatus of the type described in the exemplary embodiments include an outer casing having at one end an outlet orifice for an electrolyte and a first connection for a pressure medium. The apparatus has an inner casing in the outer casing, a first chamber between the outer casing and the inner casing, a second chamber in the inner casing in a region thereof facing away from the outlet orifice and a second connection for a pressure medium that communicates with the second chamber. The apparatus also has a nozzle pipe that leads from the second chamber to the outlet orifice, with a debouchment situated inside the first chamber behind the outlet orifice, a pump having a connecting coupling connected to at least one of the connections by a pressure medium, which pump produces at least indirectly two different operating pressures, P0 and P1, for the first chamber and the second chamber, of which at least one of the operating pressures is oscillating.